In most known methods of this type for image focusing the receivers consist of twin type photosensors, which when the image is correctly focused receive equal amounts of IR radiation. When the slide is shifted out of the focal plane to one or the other direction, the image of a mask aperture imaged on and reflected from the slide onto the two cells of the IR receiver moves across the sensor windows in one or the other direction. As a result, one of the two receivers is supplied with more IR radiation than is the other, causing an adjusting mechanism to readjust the distance between slide and lens by using the IR transmitter and receiver typically coupled to that mechanism. This method causes hardly any problems when slides of one type of mount are used, as for example glassless or glass-mounted slides using glass of identical thickness.
In the case of glass-mounted slides however, there is a problem in that when the IR light beam hits bright areas of the slide--three images are projected onto the receiver. The first image corresponds to the back reflection off the glass surface of the slide, the second image to the reflection off the film base. There are actually four images located close to one another, i.e. a first one of the rear surface of the front glass, a second one of the base material of the film, a third one of the emulsion side of the film and a fourth one of the front surface of the rear glass, all of which are imaged on the twin type receiver. If the second image is projected onto the receiver such that each of the two receivers is supplied with an identical amount of IR radiation, the slide is not yet focused. Due to the glass located in front of the slide the object plane is shifted forwards by the amount: ##EQU1## EQU d=thickness of glass EQU n=IR refraction index of the glass
whereas the autofocus system coupled to the lens was adjusted by the amount: ##EQU2## with a representing the angle between the impinging or reflected central beam of the autofocus system and the optical axis of the imaging system.
To eliminate this disadvantage, the following solutions have been disclosed, for example:
To consider the different focal planes for glass-mounted and glassless slides, in DE-OS 1 797 061 a misalignment of the autofocusing device is introduced by using an additional photosensor when glass-mounted slides are projected in which the photosensor is illuminated via a semitransparent auxiliary mirror located in front of one of the halves of the twin type photosensor. The auxiliary mirror is arranged such that the back reflection off the film layer of a glassless slide does not hit the semitransparent mirror so that no shifting of the focal plane is caused in this case, which would otherwise be effected when the auxiliary photosensor were illuminated.
From FR-PS 1 536 100, it is known that after each slide change the slide gate is moved from an extreme position to a focused position by means of an advancing device, the reflections generated by a glass-mounted slide being counted by means of a logic when they impinge on an electrooptical transducer and the advancing device being shut off when the second reflection decisive for focusing hits the transducer. It is, however, a disadvantage that each slide is initially projected when out of focus as the number of reflections has to be determined from the extreme position.